【作者】 陈海林；

【导师】 杜勇； 徐洪辉；

【作者基本信息】 中南大学 ， 材料学， 2008， 博士

【摘要】 CALPHAD（CALculation of PHAse Diagram）技术是目前非常成熟且用途极为广泛的相图评估和计算方法,它不仅是材料动力学、显微组织演变计算模拟的热力学平台,而且能够广泛地应用于新材料的设计与研制。Cu、Fe、Nb、Ni、Si、Cr、Ti等是Al基合金中的主要合金元素或添加元素。为了设计铝基合金成分、优化制备工艺,并有效地提高合金性能,有必要获得准确的多组元铝合金相图与热力学信息。作为“多组元Al合金体系的相图测定、热力学计算及显微组织演变的模拟”研究项目的一部分,本文对Al-Cr-Ti、Al-Cr-Si、Al-Cu-Fe、Al-Cu-Ni、Nb-Ni合金体系的晶体结构、相变与相图进行了系统研究:（1）本工作测定了Al-Cr二元系“γ”区域的晶体结构、相关系与相变;测定了富Al端的相平衡与转变温度,并否定了该区域内存在新相“ν”的报道;测定了62～86 at.%Al范围内的液相线。综合上述实验结果,修订了Al-Cr二元相图。采用XRD、SEM/EDX等实验手段,测定了Al-Cr-Ti体系中固溶体相（Cr）和β（Ti）,二元化合物β-TiCr₂,TiAl₃,和TiAl,以及三元相B2和τ的固溶度与相关系,从而构筑了该体系的900℃等温截面。利用平衡态合金的DTA分析,综合铸态合金的凝固组织分析,研究了合金的相变过程与凝固过程,确定了Al-Cr-Ti体系的三元零变量反应,并构筑了该体系的液相面投影图与希尔反应图。（2）通过Cr-Si二元合金的凝固组织分析,发现了Cr₅Si₃是由Cr₃Si通过包晶反应生成的,在此基础上更新了Cr-Si二元相图及其热力学优化。实验测定了Al-Cr-Si三元体系平衡相的晶体学数据;新发现了τ₂、T₃与τ₄三个三元化合物,并初步确定了τ₂与τ₄的晶体结构;利用Rietveld全谱拟合,准确地精修了τ₃的晶体结构。研究了Al-Cr-Si合金在800℃下的相平衡关系,并构筑了完整的等温截面。利用DTA测定了三元合金的相变温度,结合合金的凝固组织分析,构筑了Al-Cr-Si体系的液相面投影图与希尔反应图。（3）实验测定了Al-Cu-Fe三元体系的600℃等温截面与富Al角的相变温度。根据最新报道的相图数据,修订了Al-Fe二元体系的热力学描述。优化和计算了Al-Cu-Fe三元体系整个温度与成分范围内的相平衡,获得了一套描述各相自由能的热力学模型与参数,计算所得的等温截面与实验数据基本一致。利用同一个方程模拟了该体系中的Bcc_B2有序相与Bcc_A2（αFe,ε₁和Cu₃Al）无序相;准确地描述了A2与B2之间复杂的有序一无序转变与二级转变,以及Bcc的溶解度间隙。（4）实验测定了Al-Cu-Ni三元体系富Al角的相平衡,在实验研究的基础上,重新评估了该体系的相图数据、晶体学数据与热力学数据。通过热力学优化,模拟了Al-Cu-Ni体系中的Fcc_Al-L1₂和Bcc_A2-B2两个三元有序无序转变,获得了一套描述各相自由能的热力学参数。获得了Al-Cu-Ni体系1300～500℃等温截面、零变量反应、以及液相混合焓性质图,并构筑了该体系完整的希尔反应图。（5）对Nb-Ni二元体系进行了实验研究,证实了Ni₈Nb相的稳定存在并测定了其相转变温度。在本实验测定的基础上,重新评估了Nb-Ni体系的文献数据并进行了热力学优化。采用一种四步分步优化方法,获得了一套描述Nb-Ni二元系的自洽的热力学参数。更多还原

【Abstract】 CALPHAD technique is an effective and widely utilized method for assessment and calculation of phase diagrams.It provides a thermodynamic basis for materials kinetics and the computational simulation of the microstructure evolution.Furthermore,it can be widely applied to the design and the synthesis of new materials.Cu,Fe,Nb,Ni, Si,Cr and Ti are important alloying elements or additives in the Al-based alloys.To design alloy compositions,optimize the synthesis process,and effectively improve alloys’ performance,an accurate knowledge of the phase diagram and thermodynamics of the multi-component Al-based system is needed.As part of the project "experimental investigation, thermodynamic modeling and simulation of microstructure evolvement of multi- component Al-based alloy system",this work was involved in the investigation of the ternary Al-Cr-Ti,Al-Cr-Si,Al-Cu-Fe and Al-Cu-Ni systems.This thesis is comprised of five parts:（1）The Al-Cr binary phase diagram,especially for the crytal structure and phase equilibria in theγphase region,was experimentally determined.The phase equilibria at 900℃of the Al-Cr-Ti system were determined using XRD and SEM/EDX,and an entire isothermal section was correspondingly constructed.Based on the combination of DTA of the equilibrated alloys and the observation of microstructures of as-melt alloys,information on solidification paths and phase transitions were obtained,and therefore the liquidus projection and Scheil reaction scheme were constructed for the Al-Cr-Ti system.（2）Crystallography of the solid phases in the ternary Al-Cr-Si system was experimentally examined.Three ternary compounds,τ₂,τ₃ andτ₄ were newly found and the crystal structures ofτ₂ andτ₄ were preliminarily determined.The site occupancies ofτ₃ were determined by full profile Rietveld refinement.The phase equilibria at 800℃in the ternary Al-Cr-Si system were investigated and the entire isothermal section was determined.Based on the transition temperatures resulting from DTA of the equilibrated ternary alloys,and the solidification paths from the microstructure analysis of the as-melt alloys,the projection of liquidus surface and the Scheil reaction scheme were constructed.The microstructure evidences from the Cr-Si binary alloys yield that the Cr₅Si₃ phase forms from Cr₃Si via a peritectic reaction.The Cr-Si binary phase diagram was thus revised and its thermodynamic description was updated.（3）The isothermal section at 600℃and the phase relations at the Al-rich corner in the ternary Al-Cu-Fe system were experimentally determined.A thermodynamic modeling of the Al-Cu-Fe system was carried out by fitting the phase equilibria in the entire compositional range and the temperature range from 400 to 1300℃.The A2/B2 order-disorder transition was described by using one equation to model both ordered and disordered phases.The immiscibility gap of the bcc phase and the second order transition between A2 and B2 were well fitted. A series of isothermal sections and the projection of the liquidus surface were calculated for comparison with experimental data.（4）The phase equilibria at the Al-rich corner of the Al-Cu-Ni system were experimentally studied.Based on the experimental results,the phase diagram data,crystallographic data and thermodynamic data were evaluated once again.A set of self-consistent thermodynamic description was obtained for the Al-Cu-Ni system by reproducing all the accepted experimental data.Comprehensive comparisons between the calculations and experimental data showed that the enthalpies of mixing of the liquid were well reproduced,and the isothermal sections at 500,600,700,800, 900,1100 and 1300℃,the invariant equilibria,as well as the projection of the liquidus surface were generally fitted.A Scheil reaction scheme was proposed based on the experimental data and thermodynamic calculations.（5）The Nb-Ni binary system had been experimentally reinvestigated and the stability of Ni₈Nb was confirmed.Based on the present experimental results,the Nb-Ni system was reviewed and a thermodynamic reassessment was conducted.A set of self-consistent thermodynamic parameters for the Nb-Ni system were obatained by utilizing a four-step optimizaing procedure.更多还原